Here i suggest a method to make soft idlys  at home.

Ingredients

Raw rice 1cup
Urad Dal 1 /3cup
Puffed Rice 2 Hands full
Fenugreek Seeds ¼ tsp
Salt As per taste
Water As required
Gingely Oil AS required

Method

Wash and soak the raw rice , urad dal and fenugreek seeds in luke warm water for about 5-6 hours. Grind the mixture along with puffed rice and salt with required quantity of water for about 20 minutes in a grinder.Let it ferment for about 8 hours. Scrub the idly mould with gingely oil and steam over medium heat.

Note: The softness of the idly again depends on the variety of urad dal used.
If you require you can use cooked boiled rice instead of puffed rice.